The present invention relates to a process of treating organic wastes, and, in particular, it is concerned with a organic waste treating process which recovers methane from organic waste by the fermentation effect of anaerobic bacteria.
In the past, an anaerobic digestion process has been employed for treating organic wastes such as sewage sludge and food processing drainage, etc. This process has advantages not only in its suitability to treat wastes which have high water content or have difficulties in incineration, but also has a feature facilitating the recovery of methane as a source of clean energy, and recently active studies are being pursued for its application to city garbage and agricultural wastes. In most of such instances relating to the treatment of sewage sludge and human and livestock wastes which are in slurry form, they are being treated as they are, while in the case of city garbage and agricultural wastes they are being subjected to digestion after they are turned into slurry form by adding water thereto. In other words, in the practical sense, sewage sludge and human and livestock wastes are treated in their original slurry form, while city garbage and sewage sludge dehydrated cake are digested by so-called liquid culturing after being turned into liquid slurry by adding water thereto (e.g. in the method described in Japanese Published Unexamined Patent Application, Ser. No. 134002, Showa 53 Gazette, the subject of treatment is treated after it is turned into a slurry of 90.about.92% water content, or gruel type wastes).
According to this kind of process, however, the dimensions of the digestion tank to be employed are necessarily large due to the required dilution of the wastes by water. Therefore, much more energy is required for heating the water in the slurry up to fermentation temperature and much of electric power is required for stirring the slurry during the period of time the fermentation is being performed. Further, significant amounts of energy and expenses are required for dehydration of the digested slurry and disposal of final waste water. In addition, this approach has the additional disadvantage in that it tends to give rise to scum over the liquor surface and to sedimentation and solidification of sand and pebbles at the tank bottom.
If, therefore, these wastes could be digested in their solid state as they are, the dimension of the digestion tank could be made smaller, disposal of final waste water could be dispensed with and a wide reduction could be attained in the energy consumed. Thus, these problematic areas could be eliminated. As a result of various studies relating to solid state fermentation which does not require the turning of waste into slurry, applicants have confirmed the following points.
(1) In the case where the organic capacity load is extremely low, such as not more than 0.05 kg.VS/m.sup.3.d, fermentation is possible with or without stirring;
(2) However, if stirring is performed within the tank, the load could be increased to 0.2 kg.VS/m.sup.3.d; and
(3) If the load is set at over 0.2 kg.VS/m.sup.3.d, the waste under treatment becomes acidic and the generation of methane drastically reduced, or at worst, it stops generation.
Accordingly, the solid state fermentation would be possible if such conditions are taken into consideration, but operation under a load of around 5 kg.VA/m.sup.3.d, comparable to that of conventional slurry fermentation, becomes very difficult.
In this case (load of 5 kg.VS/m.sup.3.d), stirring the solids within the tank not only requires very much energy, as compared with stirring wastes in slurry form, but also an exceedingly large burden is placed on the digestion tank and the stirring mechanism. Therefore, not to mention the case where the objective is to conserve energy, even in the case where the waste treatment is the only objective, its practicability is judged to be low.
Generally in an anaerobic digestion, it is known that the organic matters contained in the feed wastes only decompose into methane when passed through two different fermentations, i.e. first, a liquefying fermentation wherein the organic matters are turned into such volatile fatty acids of low molecular weight as acetic acid, propionic acid and butyric acid by the effect of facultative anaerobic bacteria (liquefying bacteria or septic bacteria), and second, gasifying fermentation wherein thus generated fatty acids are converted into methane through the effect of obligatory anaerobic bacteria (gasifying bacteria or methane bacteria). The digestion process presently being practised in general is a mixed fermention method which carries out a parallel-dual fermentation within the fermentation tank under the co-presence of these two bacteria groups. In this case, the optimum pH range for the liquefying fermentation exists in the range of acidic to neutral, whereas that for the gasifying fermentation is somewhere from neutral to weak-alkaline range. For this reason, the liquefying fermentation is performed around the neutral range, but the fermentation speed of the gasifying fermentation is less than that of the liquefying fermentation. Therefore, if the operation is performed under an overloaded condition within the tank, there is the risk that the gasifying fermentation might be brought to a halt by a lowered pH within the system as the fatty acids accumulate.